In optical communications networks, optical signals are transported by optical fibres. Generally, these days, in order to maximise fibre utilisation, signals are transported as multiplexed streams, typically in dense wavelength division multiplexed format; the transport bandwidth of the fibre is divided into a number, typically forty, of evenly spaced wavelength or colour channels, and signals on each channel are combined into a multiplexed stream.
Optical signals in optical communications networks require amplification periodically to offset losses incurred in transportation. One method of amplification is with optical fibre amplifiers such as erbium doped fibre amplifiers (EDFAs). An EDFA may comprise a length of erbium doped active optical fibre; the input signal for amplification is transported along the active fibre that is also pumped with light which raises energy levels. The input signal stimulates downward transitions resulting in photons at the wavelength of the input signal, thereby amplifying it. Usually, a fibre amplifier will be located at a node in the network along with various other devices which may carry out required processes on incoming multiplexed streams.
A drawback associated with using fibre amplifiers for amplifying multiplexed streams is that fibre amplifiers tend to have a non-uniform gain profile; that is to say, gain may vary across the operable wavelength band of the amplifier. This equates to differences in amplification between the signals on successive channels in a multiplexed stream. In addition, changes in the gain profile may result from differences in power between the signals on successive channels. It is desirable to be able to tailor the gain profile to specific requirements rather than to have to make do with the gain profile of the fibre amplifier. In one specific instance, “flat gain” across the transport bandwidth is required.
There are various known methods of compensating for the non-uniform gain profile of EDFAs so as to effect a flat gain. For example, compensation may be achieved broadly across the profile by using static filters providing closely matching inverse attenuation which produce additive compensation with wavelength. However, power demands across a spectrum tend to vary, resulting in dynamic changes to the gain profile. In such circumstances, dynamic compensation is more appropriate. One dynamic compensation method involves using a small number of sequential Fourier filters, such as controlled Faraday rotators of the type disclosed in EP 634025 (Friskin et al). However, despite its dynamism, such a method does not allow each channel to be controlled individually. Moreover, optically sequential filters suffer from high optical loss for increasing channel resolution.
A device which enables the degree of attenuation on each channel within a spectrum to be continually varied, may be referred to as a dynamic gain flattening filter (DGFF). Such individual channel attenuation may be achieved by including a thermo optical attenuator on each of the output channels of a de-multiplexer and a static reflector at the de-multiplexed output plane, or the channels from the output plane may continue and be re-multiplexed. However, a DGFF with thermo optical attenuators requires a large number of heaters (one for each channel) and can lead to thermal effects on the de-multiplexer. Moreover, thermo optical attenuators have the downsides of polarisation dependence, high thermal load, high electrical dissipation.